Jack, Cleopatra and King Hassan

Merlin, Jalter, King Hassan and Musashi is one of the most famous servant and also FGO money maker.
Durable hard hitting Buster assassin is such a rare sight on FGO. With the arrival of Merlin that would make King Hassan hit Harder and tougher.
His Grand Assassin title isn't just for show mate, the fact until this day he's an assassin close to Merlin level.

While Jack stongest condition come only to female slaying, King Hassan is good for general purpose and slaying Bossess can also act as last man standing.

From gameplay purpose i would recommend Cleopatra and King Hassan.
Both are durable, Cleo is tankier, King Hassan hit Harder, both are 5 star Buster Assassin.
Though it's up to your luck.

Roll for waifu and husbando, gameplay or your heart desire do you :)

They have fairly different roles.

KH is a selfish buster memer with higher than average durability for a dps servant, and has the highest damage potential of all assassins who also gets much better when paired with Merlin.

Jack is a support/dps hybrid star generator/boss killer who outshines both KH and cleo in a vacuum or in a non-optimal team and has an anti female niche, but loses to the other two in terms of durability.

Cleo is a selfish aoe buster assassin with good durability and NP spam potential and is generally considered the best AOE assassin in the game.
Her NP hits harder than most cos it has an auto buff before it activates, so she sweeps and farms very well.

In summary:
Damage: KH > Jack >= Cleo
Boss killing: KH >= Jack > Cleo
Farming: Cleo > KH > Jack
Support utility/star generation: Jack > Cleo > KH
With optimal support: KH > Jack >= Cleo
Without optimal support: Jack > Cleo => KH